1. Field of the Invention
The present invention relates to a photodiode to be employed in an image sensor, and more particularly, to a photodiode to be employed in a CCD, a close contact type image sensor, or the like.
2. Description of the Related Art
In recent years, vigorous development is being made on a photodetector employed in, for example, a charge-coupled device (CCD) using a reduction optical system, which is used in a digital camera, a video camera, or the like and a close contact type image sensor of a one-to-one optical system.
In particular, a photodiode is a photodetector for converting light irradiated thereon into an electrical signal. By using the photodiode, an image projected on a photo-detecting surface thereof can be converted into an electrical signal.
Improvement of signal to noise ratio (SN ratio) in a photodiode is important for increase in sensitivity, and can be realized by reducing a total junction capacitance of the photodiode as will be described below.
FIG. 5A is a schematic view for explaining a structure of a conventional photodiode and shows a section of the photodiode in a direction perpendicular to a photo-detecting surface.
The conventional photodiode includes a cathode 2, an auxiliary cathode 3, a substrate 4, and an anode 5.
Of those constituents, the cathode 2 and the auxiliary cathode 3 are each formed of an n-type semiconductor, and the substrate 4 and the anode 5 are each formed of a p-type semiconductor.
A higher impurity concentration is set in the cathode 2 than in the auxiliary cathode 3 to give higher carrier (electron) concentration. A higher impurity concentration is set in the anode 5 than in the substrate 4 to give higher carrier (hole) concentration. The higher impurity concentration in the cathode 2 and in the anode 5 is for obtaining sufficient contact with a wiring made of a metal.
In general, when a junction between an n-type semiconductor and a p-type semiconductor is made, holes diffuse from the p-type region to the n-type region on a junction surface and simultaneously electrons diffuse from the n-type region to the p-type region, generating a region containing no carrier (depletion layer). In FIG. 5A, a depletion layer 10 is indicated as a region defined by dotted lines. In the depletion layer 10 a capacitor (capacitance) is formed since both carriers don't exist, leaving positive and negative impurity ions held in a crystal lattice.
FIG. 5B is a schematic view showing a capacitor formed by the depletion layer 10.
The capacitor formed by the depletion layer 10 covers an entire junction region of the p-type semiconductor and the n-type semiconductor. The entire junction region can be separated as a bottom capacitor region, which is the bottom portion of the auxiliary cathode 3, and a side capacitor region, which is the side portion of the auxiliary cathode.3.
A capacitor 21 (capacitance Ci) corresponds to the bottom capacitor region and a capacitor 22 (capacitance Cg) corresponds to the side capacitor region.
The (total) junction capacitance of the p-type semiconductor and the n-type semiconductor (total junction capacitance of the auxiliary cathode 3 and the substrate 4) is equal to the composite of the capacitances of the capacitor 21 and the capacitor 22.
The capacitor 21 and the capacitor 22 are connected in parallel to each other, so the junction capacitance Cs is expressed by the following equation (1).Cs=Cg+Ci  (1)
Further, a total diode capacitance Ct of the photodiode is the sum of the junction capacitance Cs and a cathode wiring capacitance Ch, and is therefore expressed by the following equation (2).Ct=Cs+Ch  (2)
Further, a photo-detection voltage Vs of the photodiode is determined by an amount of photocharge Qp induced by light and the total diode capacitance Ct, and is therefore expressed by the following equation (3).Vs=Qp/Ct  (3)
As is apparent from the equation (3), a large photocharge Qp or a small capacitance Ct is needed to have an increased photo-detection voltage Vs to improve the SN ratio.
JP 11-112006 A discloses a technique by which reduction in the total junction capacitance Cs reduces the total diode capacitance Ct. This document discloses a technique by which a cathode is enclosed with an auxiliary cathode having a lower impurity concentration to reduce the total junction capacitance Cs. The cathode and the auxiliary cathode correspond to the cathode 2 and the auxiliary cathode 3 of FIGS. 5A and 5B, respectively.
However, further improvement of SN ratio in the photodiode has been still demanded in the conventional technique. In particular, downsizing (area reduction) of the photodiode has been in progress, raising a need for technique of achieving a sufficient SN ratio, which can be applied to even such a downsized photodiode.